The present invention relates to methods for establishing and adapting communication link parameters, including symbol rate and transmission modulation density, in xDSL transmission systems.
Basic telecommunications services such as analog telephony, or Plain Old Telephony Service (POTS), currently serve as a main source of revenue for local exchange carriers and other telephone companies. However, there is a growing demand for high speed data services, such as digital video and high speed Internet access. Although there has been relatively little deployment of high speed data services to date, local exchange carriers and other telecommunications service providers are working to find cost-effective ways of meeting the growing demand for those services.
The part of the telecommunications network that connects a telephone central office to subscriber premises is known as the local loop. Local loops are still largely comprised of twisted wire pair links. Such links, which were originally installed to provide narrowband POTS, usually employ copper wire as the electrical transmission medium. Some twisted wire pair links have either been upgraded or replaced by the installation of fiber optic links, but the majority of local loop twisted wire pair links have been in place for many years.
Telecommunications service providers are currently exploring a number of technologies for providing high speed data services, including wireless technology, Hybrid Fiber Coax (HFC), Fiber-To-The-Curb (FTTC), Fiber-To-The-Home (FTTH), Asymmetric Digital Subscriber Line (ADSL), Symmetric Digital Subscriber Line (SDSL), High-speed Digital Subscriber Line (HDSL), and Very-high-speed Digital Subscriber Line (VDSL) technology. Although many of these technologies are likely to play some role in service providers"" long-term business strategies, many providers are seeking to meet the growing demand for high speed data services by using switched wireline infrastructures based on Fiber-To-The-Neighborhood (FTTN) and xDSL. In an FTTN local loop, optical fiber connects the central office to a cross-connection device, and the last portion of the local loopxe2x80x94from the cross-connection device to the subscriber premisexe2x80x94is comprised of a twisted wire pair link. The last portion of the local loop can also be comprised of other types of transmission facilities, as known to those of skill in the art.
As used in this specification, the term xe2x80x9cxDSLxe2x80x9d refers to ADSL, SDSL, HDSL, VDSL, and other digital subscriber line technologies known to those of skill in the art. xDSL technology enables telephone companies to provide high speed data services over local loops comprised in whole or in part of twisted wire pair links (or other transmission facilities), as in FTTN networks.
In a typical xDSL installation, the transmission facility that connects a subscriber residence to the rest of the telecommunications network is terminated by a subscriber-side xDSL termination unit (SS-XTU) on the subscriber side, and a line-side xDSL termination unit (LS-XTU) on the line or central office side. The LS-XTU receives signals from the upstream portion of the telecommunications network, and converts the signals from the format employed by the telecommunications network into xDSL format. The SS-XTU receives the xDSL-modulated signals, demodulates them, and passes them on to various telecommunications devices at the subscriber residence. The SS-XTU also transmits information in the upstream direction, where it is received by the LS-XTU, reformatted, and passed on to the telecommunications network.
In digital communication systems, the signal being transmitted is generally distorted by channel impairments, which results in intersymbol interference (ISI). A general means to reduce the error rates resulting from ISI is to use an equalizer. Thus, in most embodiments involving digital communication, the SS-XTU and the LS-XTU contain an equalizer designed to compensate for or reduce ISI.
One method of equalization is based on the use of previously detected symbols to suppress ISI in the present symbol being detected. An equalizer that implements such a method is called a decision feedback equalizer (DFE).
DFEs contain both feedback and feed-forward filters. The filter coefficients are commonly adjusted according to the mean-square error (MSE) criterion, which seeks to minimize the mean-square value of the error between the information symbol being transmitted and the estimate of that symbol at the output of the equalizer. The basic idea is to move the set of equalizer coefficients closer to the optimum set corresponding to the minimum MSE. Other criteria such as peak distortion can be used to optimize filter coefficients. Such criteria, as well as other types of equalizers and equalization methods, are known to those of skill in the art.
A typical approach in establishing communication between a LS-XTU and a SS-XTU is to transmit a signal from the LS-XTU to the SS-XTU at a predetermined symbol rate, and to attempt to determine a solutionxe2x80x94i.e., to converge the equalizer coefficients on an optimum solutionxe2x80x94for the transmitted signal. However, if the symbol rate is highxe2x80x94as it needs to be for xDSL transmissionxe2x80x94such xe2x80x9cblindxe2x80x9d acquisition may cause the equalizer solution in the SS-XTU to diverge, especially if the transmission facility between the LS-XTU and SS-XTU contains bridged taps. In those situations, communication may not be established between the LS-XTU and SS-XTU, or, if established, the communication may be poor. Consequently, there is a need for an improved method of establishing communication between xDSL line-side and subscriber-side termination units.
Before a telecommunications company can provide xDSL-based high speed data services to a subscriber, it needs to select a symbol rate and modulation density to use for communication between xDSL termination units.
Telecommunications companies are likely to provide various high speed data services, such as digital video, Internet access, ethernet service, and telephone service, which differ in terms of bandwidth requirements. The territories of telecommunication companies (or service areas within a single territory) may also differ in terms of the length and quality of the transmission facilities between the line-side and subscriber-side termination units. Individual local loops are thus likely to differ in terms of their signal-to-noise ratios and bit error rates. As a result, no single combination of a particular symbol rate and a particular transmission modulation density is likely to be optimal for all high speed data services and deployment scenarios. Consequently, there is a need for an efficient method to determine a symbol rate and modulation density combination that is capable of supporting a bit rate sufficient to provide desired high speed data services on individual local loops.
In addition, telecommunications companies need to maintain their ability to provide basic telecommunications services, such as POTS, Integrated Services Digital Network (ISDN), and Universal Digital Channel (UDC) service, which constitute primary sources of revenue, over the same facilities used to provide newer xDSL-based services. Consequently, a method for determining how to provide xDSL service should also specify parameters, such as an upstream and a downstream frequency range, that avoid or at least reduce the risk of interference with basic telecommunications services that are being provided either on the same link or on adjacent links.
The present invention provides a method for establishing and adapting communication link parameters in xDSL transmission systems. The overall method includes the steps of establishing communication between xDSL termination units, and determining a maximum transmission modulation density that can be used for each of a plurality of predetermined symbol rates. The invention provides additional, specific methods for performing those two steps: a symbol rate stepping method for establishing communication between xDSL termination units, and a rate training method for determining a maximum transmission modulation density that can be used for each of a plurality of predetermined symbol rates.
In a preferred embodiment of the present invention, the symbol rate stepping method and the rate training method are used in the context of the overall method for establishing and adapting communication link parameters in xDSL transmission systems. In alternative embodiments, the symbol rate stepping method and the rate training method can be used independently.
The symbol rate stepping method is directed to an improved method of establishing communication between xDSL termination units at a given symbol rate. Rather than transmitting a signal at the final, desired symbol rate, and attempting to determine an equalizer solution xe2x80x9cblindlyxe2x80x9d on that signal at the receiving termination unit, the present invention transmits a signal at a lower initial symbol rate, and iteratively steps up that rate by a symbol rate increment until the final, desired symbol rate is reached.
Within each iteration, after the receiving xDSL termination unit acquires a signal at a particular symbol rate, the equalizer solution that is determined for the signal at that symbol rate is held and used as the starting point for determining an equalizer solution for the signal at the next, increased symbol rate. By using this approach, the present invention reduces the risk of equalizer divergence, especially in the presence of bridged taps, and thereby provides a more efficient method of establishing communication between xDSL transmission and reception units.
More specifically, in a digital subscriber loop system comprised of a LS-XTU and a SS-XTU connected via a transmission facility, wherein the SS-XTU is comprised of an equalizer, the symbol rate stepping method of the present invention, which is directed to establishing communication between the LS-XTU and the SS-XTU at a desired symbol rate, comprises the steps of (a) setting an initial symbol rate equal to a first fraction of the desired symbol rate; (b) setting a symbol rate increment equal to a second fraction of the desired symbol rate; (c) initially setting a variable communication symbol rate equal to the initial symbol rate; (d) transmitting a signal from the LS-XTU to the SS-XTU at the variable communication symbol rate; (e) acquiring the signal transmitted at the variable communication symbol rate at the SS-XTU by determining a tentative equalizer solution for the signal transmitted at the variable communication symbol rate; (f) increasing the variable communication symbol rate by the symbol rate increment; (g) transmitting a signal from the LS-XTU to the SS-XTU at the variable communication symbol rate; (h) acquiring the signal transmitted at the variable communication symbol rate at the SS-XTU by using the previously determined tentative equalizer solution to determine a new tentative equalizer solution for the signal transmitted at the variable communication symbol rate; and (i) repeating steps (f)-(h) until the variable communication symbol rate is equal to the desired symbol rate.
The invention contemplates that the symbol rate stepping method can be used to establish communication between the LS-XTU and the SS-XTU in either the upstream direction, the downstream direction, or both.
The rate training method of the present invention is directed to determining, at each of a number of predetermined symbol rates, a maximum modulation density that can be used for communication between xDSL termination units. The maximum modulation density that is determined for each symbol rate can~then be used to compute a maximum bit rate achievable at each symbol rate. That informationxe2x80x94the maximum modulation density and the maximum bit rate achievable at each symbol ratexe2x80x94can then be used to select an optimal combination of symbol rate and transmission modulation density capable of delivering a set of desired high speed data services over a particular local loop, as described below in the method for establishing and adapting link parameters.
The first step in the rate training method is to establish communication at a predetermined symbol rate between the xDSL transmission and reception units. This step can be accomplished using the symbol rate stepping method described previously. In a preferred embodiment, the signal-to-noise ratio at that predetermined symbol rate is then measured and used to determine the maximum transmission modulation density that can be used at the predetermined symbol rate. The maximum transmission modulation density and the predetermined symbol rate can then be used to compute the maximum bit rate achievable for the predetermined symbol rate. It is also possible to determine the maximum transmission modulation density and the maximum achievable bit rate for a number of predetermined symbol rates.
The rate training method can be executed upon the occurrence of a number of events, such as when the telecommunications system comprising the xDSL transmission and reception units is initialized, when the telecommunications system software is upgraded, or when xDSL transmission and reception units are power cycled. In that way, the maximum transmission modulation density and bit rate achievable for each predetermined symbol ratexe2x80x94and ultimately the optimal combination of symbol rate and transmission modulation density capable of delivering a desired set of high speed data services over a given local loopxe2x80x94can be determined every time a specified event occurs. The execution of the rate training method upon the occurrence of a particular event potentially results in the selection of a different optimal symbol rate and transmission modulation density every time such an event occurs.
More specifically, in a digital subscriber loop system comprised of a LS-XTU and a SS-XTU connected via a transmission facility, the rate training method of the present invention, which is directed to determining a maximum transmission modulation density achievable for communicating between the LS-XTU and the SS-XTU at a selected symbol rate, comprises the steps of establishing communication between the LS-XTU and the SS-XTU using the selected symbol rate and an initial transmission modulation density; measuring a signal-to-noise ratio from the LS-XTU to the SS-XTU; and, responsive to the signal-to-noise ratio measurement, determining a maximum transmission modulation density for use between the LS-XTU and the SS-XTU at the selected symbol rate.
In addition, a maximum bit rate achievable between the LS-XTU and the SS-XTU can be computed for the selected symbol rate, responsive to the maximum transmission modulation density.
The invention contemplates that the rate training method can be used to determine a maximum transmission modulation density and a maximum bit rate achievable for communicating between the LS-XTU and the SS-XTU for a plurality of selected symbol rates, and in either the upstream direction, the downstream direction, or both.
The method for establishing and adapting link parameters of the present invention enables xDSL transmission and reception units to establish and adapt link parameters in a way that makes the delivery of both traditional telecommunications services and high speed data services possible.
The first step in the method for establishing and adapting link parameters is to select a downstream (LS-XTU to SS-XTU) spectral allocation scheme and an upstream (SS-XTU to LS-XTU) spectral allocation scheme. This may be accomplished by placing the upstream xDSL frequency range above the frequencies used by other telecommunications services on the link, and the downstream xDSL frequency range above the upstream xDSL frequency range. In addition, the upstream and downstream xDSL frequency ranges may be placed so as to minimize overlap with the frequency ranges of other telecommunication services being utilized on adjacent links.
In a preferred embodiment, the next step in the method for establishing and adapting link parameters is to determine the maximum transmission modulation density and corresponding maximum bit rate achievable for each of a plurality of preselected symbol rates. This step may be performed by using the rate training method described previously.
Finally, in a preferred embodiment, an optimal symbol rate and transmission modulation density are selected, based on the maximum transmission modulation density and maximum achievable bit rate for each preselected symbol rate. The selection of an optimal symbol rate and transmission modulation density may be based on a number of factors, including the bit rate necessary to deliver a desired set of high speed data services, and various characteristics of the deployment environment of the local loop, such as the frequencies at which impulse noise and RF ingress are likely to occur. The deployment characteristics of local loops are likely to vary by location and can be determined empirically. Because of the variance in service bit-rate requirements and deployment parameters, the optimal transmission modulation density chosen for the optimal symbol rate may be less than the maximum transmission modulation density capable of being used at the optimal symbol rate.
More specifically, in a subscriber loop system comprised of a LS-XTU and a SS-XTU connected via a transmission facility, the method for establishing and adapting communication parameters between the LS-XTU and the SS-XTU comprises the steps of (a) determining a spectral allocation scheme by selecting an upstream frequency range for communication from the SS-XTU to the LS-XTU having a lower limit greater than the upper limit of frequency ranges utilized by pre-designated telecommunications services on the transmission facility, and selecting a downstream frequency range for communication from the LS-XTU to the SS-XTU having a lower limit greater than the upper limit of the upstream frequency range; (b) for a each of a plurality of preselected downstream symbol rates within the downstream frequency range, determining a maximum transmission modulation density and a corresponding maximum bit rate achievable; (c) responsive to the maximum transmission modulation density and the corresponding maximum bit rate achievable for each of the plurality of preselected downstream symbol rates, selecting a downstream symbol rate from the preselected downstream symbol rates; and (d) selecting a downstream transmission modulation density for use with the selected downstream symbol rate that is less than or equal to the maximum transmission modulation density for the selected downstream symbol rate.
In addition, the method for establishing and adapting link parameters may be further comprised of the steps of retrieving a set of deployment parameters for the transmission facility, as well as a service bit rate requirement for the transmission facility; and selecting a symbol rate and transmission modulation density in response to the set of deployment parameters and the service bit rate requirement. The method for establishing and adapting link parameters also contemplates determining an optimum symbol rate and transmission modulation density capable of achieving the bit rate requirement while satisfying the set of deployment parameters, and selecting the optimum symbol rate and transmission modulation density as the symbol rate and transmission modulation density to use between a LS-XTU and a SS-XTU.
The invention contemplates that the method for establishing and adapting link parameters can be used to set and adapt communication parameters between a LS-XTU and a SS-XTU in either the upstream direction, the downstream direction, or both.
Another aspect of the present invention is to enable the above methods to be used in a test set apparatus. The test set apparatus is used to simulate an xDSL termination unit at a subscriber premise and to verify the line performance of a given twisted wire pair link or other transmission facility without the need to enter a subscriber""s premises or to modify an actual xDSL termination unit.
It is an object of the present invention to enable the provision of xDSL-based high speed data services over local loops comprised of twisted wire pair links or other transmission facilities.
It is a further object of the invention to provide an improved method for establishing communication between xDSL line-side and subscriber-side termination units.
It is a further object of the invention to provide a method for establishing communication between xDSL line-side and subscriber-side termination units that reduces the likelihood of equalizer divergence.
It is a further object of the invention to provide a method for determining a maximum transmission modulation density that can be used at a given symbol rate for communication between xDSL line-side and subscriber-side termination units.
It is a further object of the invention to provide a method for determining an optimal combination of a symbol rate and transmission modulation density that is capable of supporting a bit rate sufficient to deliver high speed data services over individual local loops.
It is a further object of the invention to provide a method for determining communication parameters for the provision of xDSL-based high speed data services over twisted wire pair links or other transmission facilities that avoids or at least reduces the risk of interference with basic telecommunications services such as POTS over the same links.
It is a further object of the invention to provide a test set apparatus for verifying the line performance of a given twisted wire pair link or other transmission facility without the need to enter subscriber premises or to modify an actual xDSL termination unit.
The foregoing and other features, aspects, and advantages of the present invention will become more apparent from the following detailed description, which should be read in conjunction with the accompanying drawings.